The Districts Themselves.
These cities are all built on the initial colony landings.
An also come in different forms. Some may have been Terraforming stations, sent in the early days of exploration, some could be built from Scientific Stations. So many light years from Earth these places slowly began to run themselves some forgetting where they came from.
With travel initially being very slow all Districts can be at very different levels of advancement over the years, even with the help of different alien races in the universe they have built and grown at different rates. Leading to the variation of worlds and Districts within the universe.
These are Cities built under great Energy Domes, usually constructed within them as to protect the City from the outer storms or Alien life as terraforming is carried out upon the world making it more habitable for the humans within.
Massive floating islands, built upon the waves of an oceanic world.
Great floating walk ways and buildings harbouring against the storm defiantly shipping their wares to other colonies through the universe.
While parts of these great metropolises sit above ground they run for miles and miles under the surface working to harvest all the precious metals and materials to continue to build the human colonisation of the known universe.
These are just that Hubs, usually built within each City, but also as whole Cities. Some out lying moons have hub Districts upon them they are created to allow inner planetary systems to send their wares any where in the universe via vast freighter crafts.
These tower from the surface high above the pollution of the world below. These Cities have no usable resources of their own and are some of the oldest sites in the universe for human colonisation, bring in resources and shipping out goods every where.
Every City has it's Districts.
Within each City there are Many different Districts, which like the cities they are part of make up different functions.
Hub Districts Transport goods and resources.
Industrial Districts Make Said Goods.
Mining Districts gather resources.
Harvesting Districts produce food.
Commercial Districts Trade these commodities.
Residential Districts give the inhabitants places to rest and relax.